A conventional pressurized-water reactor installation includes a reactor pressure vessel containing the core, a concrete biological shield forming a cavity within which the vessel is positioned and forming a wall surrounding the vessel, and a cylindrical pressurized coolant pipe extending from the vessel. Actually, two and usually more of such coolant pipes are involved but only one need be considered here. The shield forms a wall surrounding the pressure vessel and forming a hole of substantially larger diameter than the diameter of the pipe and through which the pipe is extended with a cylindrical space therearound. The concrete wall also forms at least one air space, and possibly two or more, between the vessel and the concrete wall, and this space or these spaces are spanned by the pipe. At least one of the air spaces is provided so that air coolant can be circulated through the space around the vessel and out through the top of the space, to reduce the thermal load on the concrete, and the cylindrical space formed around the coolant pipe, by connection with the air space, receives some of the air coolant for reducing the thermal load on the concrete forming the hole through which the pipe passes.
The above involves the problem that if the coolant pipe carrying the pressurized-water coolant, fails inside of the hole, the pressurized coolant can discharge through the hole inwardly into the air space and get beneath the bottom of the pressure vessel in its cavity, the vessel then acting like a piston receiving an upward displacement force which its positioning means are not particularly designed to resist.
Therefore, there is the problem of keeping the escaped coolant, in the event of an improbable pipeline accident in the hole, from getting into the air space around the vessel and beneath the bottom of the vessel, while at the same time, during normal reactor operation, permitting the air coolant from the air space around the vessel, to have access to the cylindrical space around the coolant pipe for outward flow therethrough.